In an uplink radio system for next-generation mobile communication, importance is attached to a high transmission power efficiency of terminals in order to expand communication areas. As a radio scheme that satisfies the requirement, a system employing a single carrier (SC) having a low peak to average power ratio (PAPR) has been under consideration. Further, in the next-generation mobile communication in which high-speed data transmission is essential, when the SC signal is used to perform high-speed data transmission, interference between symbols (multipath interference) may occur.
As a simple method for suppressing the multipath interference, a linear equalizer can be used, and a frequency domain equalizer that performs equalization processing through frequency domain signal processing so as to significantly reduce the amount of calculation processing is under examination (Non-patent Document 1). In order to calculate equalizing weight, frequency domain channel characteristics are required in the frequency domain equalizer. Thus, a method of converting a pilot reception signal directly into a frequency domain signal and estimating the channel characteristics through frequency domain correlation processing between the pilot reception signal and a pilot reference signal is being studied.
FIG. 9 shows a configuration of a channel estimation device and equalization device (frequency domain equalizer) used in a conventional radio system. The channel estimation device and equalization device include a GI (Guard Interval) removal section 101, an S/P (Serial/Parallel) conversion section 102, an FFT (Fast Fourier Transform) section 103, a reception filter 104, a channel estimation section 105, a weight calculation section 113, an equalization filter 114, an IFFT (Inverse Fast Fourier Transform) section 115, and a P/S (Parallel/Serial) conversion section 116.
An example of a radio frame format in the case where a frequency domain equalizer is used is shown in FIG. 6. A radio frame signal is composed of a plurality of pilot signal blocks or a plurality of data signal blocks. In the example of FIG. 6, a pilot signal block is placed at the head of the radio frame signal followed by a plurality of successive data signal blocks.
A GI is added to the head of each block before FFT processing in order to avoid multipath interference from a block preceding each block. As the GI, a cyclic prefix is typically used which adds the last data in each block to the head thereof.
The GI removal section 101 receives a reception signal and removes a portion of the reception signal corresponding to GI. The S/P conversion section 102 performs a serial to parallel conversion of the reception signal from which the GI has been removed by the GI removal section 101. The FFT section 103 is supplied with the reception signal that has been subjected to the S/P conversion by S/P conversion section 102 and applies NFFT (NFFT is an integer equal to or more than 2 and power of 2)-point FFT to the reception signal for conversion into a signal in a frequency domain.
The reception filter 104 limits the band of the reception signal within the frequency domain so as to shape the waveform and suppress noise. As the reception filter 104, a raised cosine roll-off filter is typically used. Although, in the configuration shown in FIG. 9, filtering of the reception signal is performed through frequency domain signal processing, the filtering may be performed prior to the processing of the FFT section 103 through time domain signal processing.
The channel estimation section 105 performs frequency domain correlation processing between a pilot reception signal and a pilot reference signal to estimate channel characteristics. The channel estimation section 105 includes a pilot reference signal generation section 106, a correlation processing section 111, and a noise suppression section 112.
The pilot reference signal generation section 106 includes a S/P conversion section 107, an FFT section 108, a transmission/reception filter 109, and a ZF (Zero Forcing)/MMSE (Minimum Mean Square Error) calculation section 110.
The S/P conversion section 107 performs a serial to parallel conversion of a pilot code. The FFT section 108 applies FFT to the pilot code that has been subjected to the S/P conversion by the S/P conversion section 107 to convert the pilot code into a frequency domain. The transmission/reception filter 109 passes a frequency domain signal of the pilot code though a transmission/reception filter. Although, in the configuration shown in FIG. 9, filtering of the frequency domain signal of the pilot code is performed through frequency domain signal processing, the filtering may be performed prior to the processing of the FFT section 108 through time domain signal processing. The processing of the transmission/reception filter 109 may be omitted in order to reduce the amount of calculation processing.
The ZF/MMSE calculation section 110 uses a signal output from the transmission/reception filter 109 to calculate a pilot reference signal used in the correlation processing.
FIG. 10 shows a configuration of the ZF calculation section 110 that calculates a pilot reference signal for use in ZF channel estimation. The ZF calculation section 110 includes a square calculation section 121, an inverse number calculation section 122, and a multiplication section 123. A pilot reference signal X(m) (1≦m≦NFFT) of a sub-carrier m required for performing the ZF channel estimation is represented by the following equation.
[Numeral 1]
                              X          ⁡                      (            m            )                          =                              C            ⁡                          (              m              )                                                                                        C                ⁡                                  (                  m                  )                                                                    2                                              (        1        )            
where C(m) is the output signal of the transmission/reception filter 109.
FIG. 11 shows a configuration of the MMSE calculation section 110 that calculates a pilot reference signal for use in MMSE channel estimation. The MMSE calculations section 110 includes a square calculation section 121, a noise addition section 124, an inverse number calculation section 122, and a multiplication section 123. A pilot reference signal X(m) (1≦m≦NFFT) of a sub-carrier m required for performing the MMSE channel estimation is represented by the following equation.
[Numeral 2]
                              X          ⁡                      (            m            )                          =                              C            ⁡                          (              m              )                                                                                                            C                  ⁡                                      (                    m                    )                                                                              2                        +                          σ              2                                                          (        2        )            where σ2 is noise power.
The correlation processing section 111 is supplied with the pilot reference signal X(m) and pilot reception signal, the band of which has been limited by reception filter 104 and performs correlation between them for each sub-carrier to estimate frequency domain channel characteristics. A channel estimation value H (m) (1≦m≦NFFT) of a sub-carrier m is calculated according to the following equation.
[Numeral 3]H(m)=X*(m)PRX(m)  (3)
where PRX (m) is the pilot reception signal, the band of which has been limited by reception filter 104, a suffix * is a complex conjugation. In the ZF channel estimation, code characteristics of the pilot reception signal can be canceled, together with the characteristics of the transmission/reception filter, whereby only the channel characteristics H (m) can be detected. However, if the size of the frequency domain signal of the pilot code is not constant, noise enhancement occurs, degrading the channel estimation accuracy.
FIG. 7 shows gain characteristics (1/|C(m)|2 characteristics) of the pilot reference signal obtained in the case where a random code is used as the pilot code. In the inherent characteristics of a code and at the edge of the band, when the gain is greater than 0 dB due to attenuation of the transmission/reception filter, noise enhancement occurs. In the MMSE channel estimation, in order to suppress the noise enhancement, the gain of the pilot reference signal is determined such that the mean square error of the channel estimation value becomes minimum, which improves the channel estimation accuracy as compared with the case of the ZF channel estimation.
The noise suppression section 112 suppresses the noise of the channel estimation value estimated by the correlation processing section 111 to thereby improve the ratio of signal power to noise power (S/N). The noise suppression section 112 may employ a method of averaging adjacent sub-carriers, a method of temporarily converting a channel estimation value into an estimation value in a time domain to remove a noise path, or the like.
The weight calculation section 113 is supplied with the channel estimation value in the frequency domain which is output from the channel estimation section 105 and calculates an equalization weight in accordance with an MMSE method, in general. An MMSE weight W(m) (1≦m≦NFFT) on a sub-carrier m is calculated using the channel estimation value H(m) according to the following equation.
[Numeral 4]
                              W          ⁡                      (            m            )                          =                              H            ⁡                          (              m              )                                                                                                            H                  ⁡                                      (                    m                    )                                                                              2                        +                          σ              2                                                          (        4        )            
The equalization filter 114 is supplied with the equalization weight calculated by the weight calculation section 113 and reception signal, the band of which has been limited by the reception filter 104 and equalizes, in the frequency domain, the reception signal by multiplying the reception signal by the equalization weight for each sub-carrier. Assuming that data reception signal, the band of which has been limited by the reception filter 104 is DRX(m) (1≦m≦NFFT) and the weight calculated by the weight calculation section 113 is W (m), a signal Y(m) (1≦m≦NFFT) equalized by the equalization filter 114 is represented by the following equation.
[Numeral 5]Y(m)=W*(m)DRX(m)  (5)
where a suffix * is a complex conjugation.
The IFFT section 115 is supplied with the equalized signal in the frequency domain output from the equalization filter 114 and applies NFFT-point IFFT to the equalized signal for conversion into a signal in the time domain. The P/S conversion section 116 performs a parallel to serial conversion of the signal in the time domain so as to output it as a demodulated signal.
Non-patent Document 1: D. Falconer, S. L. Ariyavisitakul, A. Benyamin-Seeyar, and B. Eidson, “Frequency Domain Equalization for Single-Carrier Broadband Wireless Access,” IEEE Commun. Mag., vol. 40, no. 4, pp. 58-66, April 2002.